This study leverages our leadership in the evaluation of a major World Bank (WB)-funded project in four urban areas of Mozambique. We will test if and how improvements in water supply reduce or delay enteropathogen infection, and impact associated acute and chronic gastrointestinal-related disorders. We take advantage of the randomized allocation of access to a new piped water system, adding novel microbial and health endpoints. This will be the largest water system improvement evaluated using an experimental design and the first to examine the impact of new household water connections on these health outcomes. While improved case treatment has reduced mortality from diarrhea in recent decades, morbidity has persisted, with an estimated four billion diarrhea cases annually worldwide. The deleterious health effects of repeated, cumulative enteric pathogen infections and diarrhea episodes are especially relevant when they occur during the first two years of life. Increasing evidence suggests that early infection with enteric pathogens, even in the absence of diarrhea symptoms, is associated with long-term chronic sequelae, including environmental enteric dysfunction (EED), growth faltering, and cognitive impairment. Studies of WASH interventions often focus on reducing diarrhea, and few have examined specific enteric pathogens, nor long-term gastrointestinal-related sequelae. The study of host-level gastrointestinal conditions, such as gut microbiome composition and EED, may help explain differential acute responses to enteric infections, as well as the downstream consequences of these infections. While there is evidence of differences in gut microbiome composition across different cultures, regions, and populations, to date the environmental determinants of these differences, such as access to piped water, have not been evaluated experimentally, using explicit counterfactuals. More research is needed to understand these relationships and how they are affected by community-level interventions such as water supply provision. We propose to test how provision of piped water to low-income, urban households impacts: (SA1) acute gastrointestinal conditions (enteric pathogen prevalence, diarrhea prevalence); (SA2) chronic gastrointestinal conditions (EED, gut microbiome composition, growth faltering); and (SA3) timing of enteropathogen seroconversion and the association between seroconversion and chronic gastrointestinal conditions. Our multi-disciplinary team has expertise in impact evaluation, enteric diseases, drinking water quality, and metagenomics, as well as extensive experience researching enteric diseases in low-income settings throughout the world. This research will inform decisions on how best to invest resources to provide essential infrastructure to cope with urban growth globally and improve child health, especially for the estimated 1.1 billion people worldwide who lack safe drinking water supplies.